The present invention relates to a special refractor, that is to say, a flat plate consisting of various flanked dioptric elements of transparent material with refractive index higher than 1, characterized by the fact that the elements making up the system have an afocal profile of asymmetrical geometry, whereby said refractor has the capacity, although remaining fixed with time, to concentrate the luminous rays of the solar source onto a surface of considerably smaller dimensions lying beneath the refractor.
This refractor represents considerable progress in its particular field.
The capacity of Fresnel type lenses to concentrate a beam of parallel rays towards its focal point has been well known for some time now. However, such a lens performs this function if it positioned so as always to remain at right angles with time to the incoming direction of the sun's rays.
For this purpose, the Fresnel lenses are mounted on so-called "tracking" or "sun follower" mechanisms which move the lens in an appropriate manner.
In fact, owing to the apparent movement of the sun, a fixed refractor of the concentrating type transmits a solar image which, over a period of time, assumes such dimensions so as to drastically compromise the possibility of collecting solar energy with a fixed absorber.
Also well known in the art are applications of said lenses in the utilization of solar energy, wherein they are mounted on sun follower mechanisms or else they can be fixed, but in this latter case, they are coupled to suitable elements designed to substitute the presence of the sun follower mechanisms.
The refractor in accordance with the invention has been devised as stated above, and succeeds in accomplishing a spatial concentration of the solar energy transmitted by keeping its position unaltered with time, even without the aid of additional elements for correcting the direction of the luminous rays refracted by the refractor profile.
More precisely, it has been found that, although the position and inclination of the refractor are kept fixed during the daily path of the sun across the sky in such a manner as to render the use of sun follower mechanisms unnecessary throughout the day, this refractor succeeds, at least for a period of 6 hours, in concentrating the solar energy striking said refractor, on to an absorber of considerably smaller dimensions than the exposed surface area, with a concentration ratio equal to or greater than 2.
For the concentration ratio in this case, we mean the ratio between width L of the refractor surface exposed to the sun and the corresponding dimension L.sub.1 governing the surface struck by the solar image during the period of time considered.
Said profile can be advantageously used in harboring solar energy in the most suitable forms each time it is desirable to avoid the complication of having sun followers or systems for correcting the direction of the refracted rays, and when it required to transfer the energy striking a surface of arrestment to a collecting surface of smaller dimensions.
The refractor in accordance with the invention, as stated previously, consists of Fresnel type elements, either standard or modified version, said elements being made of any transparent material and arranged asymmetrically about a preselected axis as its profile does not allow an axial symmetry.
The so-designed profile does not have any single focal point as the elements making up the refractor do not have the same focal length along the axis of separation between the upper elements and the lower elements, that is to say, by keeping the refractor position fixed, a greater concentration of the sun's rays can be accomplished.
The central element, that is the element joining the respective refractor about the preselected axis, is afocal because the half elements comprising it, are of the flat-convex lens type with different focal points.
In fact, the central element actually consists of one single element formed by two different half lenses.
It has been found that by exposing said flat refractor to the sun's rays with suitable positioning of the profile with respect to the horizontal plane, transmissions of the solar image in the direction of the len's profile is such that there exists a plane in the half-gap opposite that of the incoming sun's rays, in which the incident solar energy is concentrated over an area considerably smaller than that of the collector profile.
It has also been found that said collecting area is always smaller than that which would be obtained using a conventional type Fresnel refractor of the same overall dimensions.
The lower concentrating power of the elements closer to the horizontal plane at ground level, with respect to that of the elements further away from the same horizontal plane, succeeds in limiting the excursion of the sun's rays on the collecting surface during the day.
When a refractor of symmetrical profile is used, for example, of the Fresnel type, it is possible, depending on the refractor design features, to find a plane in which the solar image strikes a surface area smaller than the area exposed to the sun over a considerable period of time. More specifically, when the angle formed by the axis of the refracted rays with the horizontal plane is such that the sun's rays strike the exposed surface perpendicularly at midday, then the width of the surface struck by the solar image during its movement is determined in the downwards direction by the rays refracted from the upper elements during the hours at the beginning and end of the time period considered, while the same direction upwards is determined by the same elements through the rays refracted at midday.
The asymmetrical profile, to which the herein disclosed invention refers, thanks to the higher dioptric power of the lower elements with respect to that of the upper elements, succeeds, while limiting the excursion of the rays refracted in downwards direction by the upper elements, in increasing the deviation of the rays refracted in the upwards direction by the lower elements. In this way, a considerably higher concentration ratio is obtained on the plane of maximum concentration with respect to that obtained on the plane of maximum concentration of a symmetrical refractor having the same overall dimensions as the asymmetrical refractor with which comparison is made.
For example, it was found when comparing a conventional Fresnel lens with a refractor of the herein disclosed type and of the same overall dimensions, that a concentration ratio of 1.59 is obtained on the plane of maximum concentration in the case of the Fresnel type lens, while for the asymmetrical refractor this concentration ratio was greater than 2.
The asymmetrical refractor can, in accordance with the required exposed surface and the solar image characteristics to be obtained, consist of an arbitrary number of upper and lower dioptric elements.
The profile of the various elements can be of the standard Fresnel type or be of a modified type.
A preferred embodiment of the refractor in accordance with the invention is illustrated in the accompanying drawings which are given as an exemplification of the principles of the invention, with no limitation to be inferred.